This application was originally deposited on Aug. 6, 2001, in the United States Patent and Trademark Office under the Disclosure Document Deposit Program and was assigned Disclosure Document No. 497,934.
This invention relates generally to the art of alloys and more particularly to a high chromium, nitrogen bearing alloy having high corrosion resistance. The instant invention also relates to a high chromium-nitrogen bearing castable alloy, a high chromium-nitrogen content alloy, and a process for producing the high chromium-nitrogen bearing alloy, and articles prepared from the same. This invention further relates to a corrosion resistant high chromium, nitrogen bearing austenitic alloy which is also excellent in strength at high temperatures and suitable for materials of boilers, chemical plant reactors and other apparatus which are exposed to severely high temperature and corrosion environments at work. The instant invention is also directed to a heat resistant high Chromium, nitrogen bearing austenitic alloy having high strength and excellent corrosion resistance in high temperature corrosive environments. The present also addresses the problem of creating a metal casting material, the wear resistance of which will correspond approximately to common commercial types of white iron, but which additionally will be characterized by high corrosion resistance in aggressive media. In addition to high corrosion and wear resistance, the alloy material according to the invention has good casting characteristics. Consequently it can be produced in conventional high-grade steel foundries. Moreover, the casting material has good working characteristics. Furthermore, the aforementioned positive quantities are primarily a chromium content of 28 to 48 wt. %, a carbon content of 0.3 to 2.5 wt. %, and a nitrogen content of 0.01 to 0.7% which result in a sufficiently high volume proportion of carbides and nitrides. The large increase of the chromium content decreases the chromium depletion of the matrix. With regard to the combination of corrosion resistance and wear resistance, the material according to the invention is decidedly superior compared to the known types of castings previously utilized in applications subjected to hydroabrasive wear. The present invention is also directed to an air-meltable, castable, workable, alloy resistant to corrosion and acids such as sulfuric acid and phosphoric acid over a wide range of acid strengths.
Equipment used in highly corrosive environments typically is constructed of metal alloys such as stainless steel or other high alloys. These alloys are necessary to withstand the extremely corrosive effects of environments in which the equipment encounters chemicals such as concentrated sulfuric acid or concentrated phosphoric acid. A particularly difficult environment is encountered in making phosphate fertilizer. In the digestion of phosphate rock with hot, concentrated sulfuric acid, equipment must resist the environment at temperatures up to about 100xc2x0 C. The impure phosphoric acid which is produced can be extremely corrosive and contains some residual sulfuric acid. The corrosive effect is often increased by other impurities in the phosphoric acid, particularly by halogen ions such as chloride and fluoride, which are normally present in the phosphate rock feedstock used in the process. An extremely corrosive environment is encountered in the concentration of the crude phosphoric acid.
Phosphate rock deposits at various locations in the world vary greatly in chemical composition. The most severe corrosion environments are typically encountered in processing deposits of phosphate rock which contain a high content of halogens, such as chloride or fluoride.
It is also generally known that increasing the Cr content is effective to improve corrosion resistance of steel. Hi-Chrome alloys containing 23-40% Cr, 0.8-2% C, 2.5% Si, and up to 5% Mo, have been known since the 1930""s. See for Example German Patent No 7,001,807. U.S. Pat. No. 5,252,149 represents a modernization of this alloy, followed by the German Patent No. 8,612,044 or No. 4,417,261. It is noted that in both patents the alloys exhibit a high resistance to abrasion and good resistance to corrosion. However, both exhibit poor mechanical properties, especially low toughness, brittleness, sensitivity to heat, sensitivity to notch all of which limit their usefulness. It is evident that their structure contains ferrite (Fe xcex1).
The ferritic structure in these alloys is inherently very brittle, and the carbide phase embedded in such a brittle phase, results in a very low toughness, high notch sensitivity, as well as sensitivity to heat. Besides, the ferritic structure supersaturated with Chrome, causes the creation of the sigma phase, which drastically lowers toughness and corrosion resistance.
U.S. Pat. No.5,320,801 is directed to alloys having the following composition: Crxe2x80x9427 to 34% by weight, Ni+Coxe2x80x9413 to 31%, Sixe2x80x943.2 to 4.5%, Cuxe2x80x942.5 to 4%, Cxe2x80x940.7 to 1.6%, Mnxe2x80x940.5 to 1.5%, Moxe2x80x941 to 4%, and Fexe2x80x94essentially the balance. The alloy of the ""801 patent possesses good toughness, but has very poor hardness and very poor wire resistance and low tensile strength. The hardness of 208 to 354 HB, is similar to that of CD4MCU stainless steel (260-350 HB), which has excellent corrosion resistance, but poor wear resistance. The alloy disclosed and claimed in U.S. Pat. No. 5,320,801 is similar to austenitic, high Nickel stainless steels in that is has good toughness, but very low tensile strength and hardness, as well as poor wear resistance. The Nickel present in corrosion resistant alloys, serves mainly for structural stabilization and adds very little to their corrosion resistance. Good examples of this are the stainless austenitic steels containing 12-35% Ni, which have corrosion resistance approaching that of duplex stainless steels which have a low percentage of Nickel (4-8%), or High-Chrome stainless steels with Ni only up to 4%. The primary elements of stainless alloys are Chromium, Molybdenum and Nitrogen as illustrated in the models used to show how various alloying elements influence the corrosion resistance of stainless steel. For example: Pitting Resistance Equivalent Number, PREN=% Cr+3.3*Mo+16*% N illustrates that Nitrogen is an important, very powerful alloying element of corrosion resistant alloys.
The main flaw of the High-Chrome alloys of the prior art is the difficulty in dissolving of Chrome, Molybdenum and Nitrogen in the matrix, without a negative effect on the mechanical properties of the alloy, such as toughness, tensile strength, brittleness, heat sensitivity and weld ability. This is the result of the precipitation of the sigma phase from alloys saturated with Chrome and Molybdenum. Premature wearing out of pump parts made from the above-mentioned High-Chrome alloys is a common occurrence. The main contributing factors are: very low toughness, brittleness and low endurance. Most often a failure happens with a casting worn thin in an isolated area where, due to the poor mechanical properties of the alloy, a crack develops leading to the eventual disintegration of the otherwise still viable component.
The mechanism for corrosion and erosion in acidic environments of the alloys of the prior art are accelerated corrosion due to the continuous removal of the passive corrosion resistant layer by particles in solids containing corrosive fluid. This is especially evident in alloys containing a higher volume of Chrome and Molybdenum, where significant amount of sigma phase is unavoidable and the metal matrix possesses very poor toughness. In order to restore the passive layer, it is necessary to have the Chrome and the Molybdenum concentration at as high a level as possible.
Increasing the Chrome/Carbon, or Cr+Mo/C ratio, increases corrosion resistance up to the critical point, after which begins the formation of the sigma phase, which drastically reduces the toughness and lowers the corrosion resistance of the alloy by depleting the Chrome in the vicinity of the sigma phase precipitates.
The present invention is based on increasing the ratio expressed by Cr+N/Cxe2x88x92N, or Cr+Mo+N/C and Cr+Mo+N+B/Cxe2x88x92N by reducing the Carbon in the matrix, while introducing the Nitrogen as a powerful additional alloy element to the High-Chrome alloys where it is in a high concentration in solid solution.
Nitrogen, like Carbon, forms interstitial solids with body-centered-cubic (bcc)-xcex1 Iron, and face-centered-cubic (fcc) xcex3-iron. The size of the Nitrogen atom is smaller than that of the Carbon atom; in this case, in the xcex1, as well as in the xcex3 phases, the Nitrogen occupies the interstitial sites easier.
The maximum solubility of Nitrogen in Fe-xcex4 and Fe-xcex3 is several times, to tens of times higher than that of Carbon at the same temperatures, which leads to significant expansion and distortion of elementary lattices. It has a solid solution hardening and strengthening effect much greater than that of Carbon, while maintaining a greater level of toughness.
The solubility limits of Nitrogen in the prior art High-Chrome alloys are a very low 0.15% N maximum. This limit is dictated by an inherently low physico-chemical solubility of Nitrogen and Carbon (0.02 to 0.08 max. C+N) in the structure Fe-xcex1, which constitutes up to a maximum of 40% of the alloy in German Patent Nos. 4,417,261 or 8,612,044, as well as the low Manganese content xe2x89xa61.5%.
The addition of Nitrogen is the most effective means of improving the mechanical properties of austenitic High-Chrome alloys without having a deleterious effect on ductility and corrosion resistance. In order for Nitrogen to be fully effective as an anti-corrosive agent, and to bring to bear its wide range of positive effects on the castings"" mechanical properties, such as increased tensile strength hardness and toughness, without loss of ductility, Applicant discovered that in High-Chrome alloys this can happen with considerable presence of Manganese and Molybdenum as enhancing alloys. In these conditions, Nitrogen dissolves in the solid state, two to four times better than in any other High-Chrome alloy disclosed in the prior art. Similarly in high Manganese stainless steels, which dissolve up to 0.8% Nitrogen, and even 1% under partial to pressure, the tensile strength and the hardness are two to four times higher, with good ductility than in the same steel without nitrogen.
The prior art is silent regarding the high-chromium alloys of the instant invention.
It is an object of applicants"" invention to produce a material of construction suitable for use in processing such phosphate rock which presents a severely corrosive environment.
It is also an object of applicants"" invention to produce a corrosion resistant alloy which is high in chromium content and which has an enhanced corrosion resistance.
It is a further object of applicants"" invention to produce a highly corrosion resistant alloy which contains silicon in sufficient quantity to render the alloy castable by conventional methods.
It is another object of applicants"" invention to produce a highly corrosion resistant alloy which contains silicon.
Still a further object of applicants"" invention is to produce a corrosion resistant alloy that is high in chromium content and also contains nitrogen.
It is an additional object of applicants"" invention to produce a corrosion resistant alloy which has high strength and hardness properties.
An additional object of the present invention is to provide a High-Chromium, Nitrogen bearing alloy with significant improvement in mechanical properties.
Yet, another object of the invention is to provide a high-chromium, nitrogen bearing alloy having greater resistance to corrosion combined with erosion, particularly in acidic environments containing chlorides, fluorides media, or other impurities.
A further object of the present invention is to provide a High-Chromium, Nitrogen bearing alloy containing a large amount of Nitrogen
It is a further object of the present invention to provide novel method of hardening a High-Chromium, Nitrogen bearing alloy by cryogenic treatment.
It is an additional object of applicants"" invention to produce a High Chromium, Nitrogen and Boron containing alloy which is erosion and corrosion resistant.
The instant invention is also directed to a corrosion and erosion resistant high-chromium nitrogen bearing and castable alloy comprising the following composition in wt. %:
28% to 48% Chromium
0.01% to 0.7% Nitrogen
0.5% to 30% Manganese
0.3% to 2.5% Carbon
0.01% to 5% Boron
optionally 0.01% to 6% Molybdenum
optionally 0.01% to 5% Silicon
optionally 0.01% to 8% Copper
optionally 0.01% to 25% Nickel and Cobalt
said alloy further containing up to 2% of each of one or more micro-alloying elements selected from the group consisting of: zirconium, vanadium, cerium, titanium, tantalum, tungsten, aluminum, niobium, calcium and rare earth elements with the balance being essentially iron and other trace elements or inevitable impurities and having a microstructure comprising chromium carbides, borides and nitrides in an austenitic matrix, said matrix being of face center cubic crystal structure, super saturated by nitrogen in solid solution form and wherein the austenicity of said alloy is defined by the following ratio                     %        ⁢                  xe2x80x83                ⁢        Ni            +              %        ⁢                  xe2x80x83                ⁢        Co            +              0.5        ⁢                  xe2x80x83                ⁢                  (                                    %              ⁢                              xe2x80x83                            ⁢              Mn                        +                          %              ⁢              Cu                                )                    +              30        ⁢                  xe2x80x83                ⁢                  (                                    %              ⁢                              xe2x80x83                            ⁢              N                        +                          %              ⁢                              xe2x80x83                            ⁢              C                                )                    +              5        xc3x97        %        ⁢                  xe2x80x83                ⁢        B                            xe2x80x83            ⁢                        %          ⁢                      xe2x80x83                    ⁢          Cr                +                  %          ⁢                      xe2x80x83                    ⁢          Mo                +                  %          ⁢                      xe2x80x83                    ⁢          Si                +                  1.5          ⁢                      xe2x80x83                    ⁢                      (                                          %                ⁢                                  xe2x80x83                                ⁢                Ti                            +                              
                            ⁢                              %                ⁢                                  xe2x80x83                                ⁢                Ta                            +                              %                ⁢                                  xe2x80x83                                ⁢                V                            +                              %                ⁢                                  xe2x80x83                                ⁢                Nb                            +                              %                ⁢                                  xe2x80x83                                ⁢                Ce                            +                              %                ⁢                                  xe2x80x83                                ⁢                Al                                      )                                ≥      1.5    .  